1. Field of Invention
The present invention relates to a method for forming such members made of metal plates as used for car parts and the like, and in particular, relates to a method for improving a defective shapes caused by a change of an angle of bent portion of the formed member, caused by elastic recovery when removed from a forming die.
2. Description of Related Art
In recent years, a body or a part for automobiles is increasingly stronger and lighter for mileage improvement, environmental consideration, and safety improvement. One way to achieve stronger and lighter parts is to use lighter materials such as high-tension steel or aluminum based alloy for a press-formed member of metal plates that takes a large portion of car component parts.
FIG. 1 shows a representative example of the press-formed member. FIG. 1A shows an L-shaped member formed by one edge of an upper flat portion 51 and a longitudinal flat portion 53 forming the member perpendicularly through a bent portion 52.
FIG. 1B shows a U-shaped member where the L-shaped members are placed symmetrically. FIG. 1C shows a Z-shaped member formed in a manner that on one edge of an upper flat portion 61, a longitudinal flat portion 63 is formed perpendicularly through a bent portion 62, and on the other edge thereof, a flange portion 65 is coupled through a bent portion 64 so as to be parallel to the upper flat portion 61. FIG. 1D shows a hat-channel shaped member where the Z-shaped members are placed symmetrically. These formed members are formed integrally of a metal plate by relatively displacing a forming die and a forming punch (sometimes called xe2x80x9cbending bladexe2x80x9d) having a forming surface corresponding to a shape of the formed member.
A problem with a press forming of a metal plate of high tension steel or aluminum based alloy is that large springback caused when a plate is removed from the forming die, deteriorates dimensional accuracy of the formed member. Examples of the above are as follows:
FIG. 2A is a diagram in which chain double dashed lines show a shape of a U-shaped member before removed from the die (i.e., a target forming shape) and solid lines show a shape of the member after removed from the die; likewise,
FIG. 2B is a diagram in which chain double dashed lines show a hat-channel shape of a member before removed from the die (i.e., a target forming shape) while solid lines show a shape of the member after removed from the die. In both cases, an angle change defect xcex94xcex8 occurs at the bent portions 52 and 62, respectively, after the member is removed from the forming die. Note that the angle change defect xcex94xcex8 is an angle formed between a tangent of the end of the bent portion on a longitudinal flat portion side and a longitudinal flat portion of the target forming shape. In the case of the hat-channel shaped member (the Z-shaped member), in addition to the angle change defect of the bent portion 62, the longitudinal flat portion 63 receiving bending for bending-back force warps suffers from an outward curvature. In the figure, xcex94 represents a maximum dividing distance between the warped portion and a line segment coupling between the end of the bent portion on the both sides of the longitudinal flat portions. The bending for bending-back force means that when a plate material bent and formed, the bent portion bends back to an opposite direction to that was bent first, and the portion that was bent back is distorted by rebounding to the first bending direction. The angle change defect and the curvature causes an opening, with an opening distance xcex94W, to be generated at a lower end of the longitudinal flat portion of the hat-channel shaped member. The opening distance xcex94W is a horizontal distance between the lower ends of the respective longitudinal flat portions of the forming shape just removed from the die and the target forming shape. The lower end of the longitudinal flat portion of the hat-channel shaped member (and the Z-shaped member) refers to an intersecting portion extended from an inner surface of the longitudinal flat portion and a lower surface of the flange portion.
Some examples of various methods of preventing angle change defect at the bent portion of the formed member are proposed as follows.
First is a method for designing a shape of a forming die so that a formed member becomes the right size when in springback;
Second is a method in which a reverse bending radius portion having an opposite direction of the bending direction is formed along a ridge line of a bent portion, described in JP-A-7-204743.
Third is a method for giving compression stress in a direction of a plate thickness of a bent portion so as to reduce the remaining stress, described in JP-A-8-174074. Incidentally, the similar methods can be basically applied to reducing the wall curvature xcex94.
Problems of those methods are as follows. In the first method, it is difficult to design a forming die (a tool) in an appropriate shape, requiring more time to try and learn, which lead to increase a cost of the forming die and to delay to start producing the product. In the second method, the reverse bending radius portion needs to be added to the bent portion of the product, causing to change an appearance of the product to a poor one.
Incidentally, the reverse bending radius portion is the one that is not originally needed. In the third method, there needs additional machines other than a pressing machine, such as a device to add compressive force.
Under the circumstances, the present invention aims at solving the problems and providing a method for bending a metal plate that can reduce an angle change defect of the bent portion of the formed member with no need of changing product shape or of preparing for a special forming die or a facility.
First aspect of the present invention resides in a method for bending a metal plate, comprising:
a first forming step for obtaining a first formed member having a bent portion obtained by bending a metal plate; and
a second forming step for bending the bent portion of the first formed member in the same direction as a bending direction of the bent portion, so that the bent portion of the first formed member receives bending-back force.
Second aspect resides in a method for bending a metal plate, comprising:
a first forming step for bending a metal plate by relatively moving a first forming punch from a first flat forming portion to a first forming die with a second flat forming portion formed therein, through a bent forming portion having a radius R1, to thereby obtain a first formed member in which a first flat portion and a second flat portion is coupled with each other through the thus formed bent portion by the bent forming portion; and
a second forming step for relatively moving a second forming punch from the first flat forming portion to a second forming die with the second flat forming portion formed therein, through a bent forming portion having a radius R2, to thereby bend the first formed member by the bent forming portion of the second forming die;
the second forming step further comprising a substep for placing the second flat portion of the first formed member on the second flat portion side of the second forming die, so that the bent portion of the first formed member receives the bending-back force while abutting with at least one of the bent forming portion and the second flat forming portion, both of the second forming die.
In the second aspect, it is preferable that in the second forming step, a part of the bent portion of the first formed member abuts with a bend stop point on a second flat forming portion side of the bent portion of the second forming die.
Further, it is preferable that the method performs the second forming step using following formulas (A) and (B) in which k is obtained from the formula (A), and then xcex94W is obtained from the formula (B) by the thus obtained k,
xe2x80x83k=xe2x88x920.4836xc2x7ln(R2/R1)+0.1817xe2x80x83xe2x80x83(A)
xcex94W=(xcfx80/2xe2x88x921)R2+(1xe2x88x92(1xe2x88x92k)xc2x7xcfx80/2)R1xe2x80x83xe2x80x83(B)
wherein the formulas assume the first formed member to be formed along the first flat forming portion of the first forming die, the bent forming portion, and the second flat forming portion;
a symbol xcex94W denotes a distance between the second flat portion of the first formed member and the second flat forming portion of the second forming die; and
a symbol k denotes a proportion of the bent portion of the first formed member abutting to the second flat forming portion of the second forming die, to the total length of the bent portion.
Furthermore, it is preferable that the R1 is set to R1xe2x89xa61.5R2 when the R2 is 5 mm or more, and to R1 greater than 1.5R1 when the R2 is less than 5 mm.
In the double bending method according to the present invention, the bent portion of the metal plate formed after the first forming step receives the bending-back force during the second forming method, and a bending moment in a direction opposite to the bending moment generating to the bent portion formed in the second forming step, thereby enabling to reduce the angle change defect of the bent portion formed in the second forming step. According to the method of bending of the present invention, forming can be performed by using a press or forming die with ease, thereby having superior productivity.
Third aspect resides in a method for bending a metal plate, comprising:
a step of preparing a forming die and a forming punch, said forming die having a forming surface in which a horizontal flat forming portion and an inclined flat forming portion being coupled with each other through a bent forming portion, said forming punch being used for bending a metal plate having an inclined flat forming portion parallel to the inclined flat forming portion of the forming die while collaborating with the inclined flat forming portion of the forming die; and
a step of bending the metal plate held at the horizontal flat forming portion of the forming die, by using the bent forming portion and the inclined flat forming portion, in such a way that the inclined flat forming portion of the forming punch relatively moves closely or away to/from the inclined flat forming portion of the forming die;
wherein in this bending step, an angle formed between the horizontal flat forming portion of the forming die and the inclined flat forming portion is to be obtuse so that the bent arch portion of the metal plate, bent round the bent forming portion of the forming die, abuts simultaneously with both the inclined flat forming portion of the forming punch and the inclined flat portion of the forming die.
In the above-described method, the method uses the forming die that has predetermined xcex8p and rp so as to have xcex94xcex8p within an acceptable range, xcex94xcex8p being obtained as a function xcex94xcex8p of xcex8p, rp, H, TS, t in advance,
where TS denotes tensile strength of a metal plate; t, a thickness of the metal plate; rp, radius of a bent forming portion of the forming die; xcex8p, an angle formed between the horizontal flat forming portion of the forming die and the inclined flat forming portion; xcex94xcex8p, a difference between the xcex8p and an angle formed between the horizontal flat forming portion and an inclined flat portion of the formed member after bending; and H, a vertical distance from the horizontal flat portion to the lower end thereof.
The function of xcex94xcex8p, a formula (1) given below may be used, and said xcex8p is preferably set to be 110xc2x0 or more, and the H is preferably set to be 30 mm or more.
The method uses, as a function of xcex94xcex8p, a formula (1) given below:
xcex94xcex8p=TS/t{cxe2x88x921xc2x7H(b1xc2x7TS/t+b2)xc2x7(xcex8pxe2x88x9290)n}xc3x97{1+exp(dxc2x7rp)}xc2x7(rp+0.5t)/{fxc2x7(5+0.5t)}xe2x80x83xe2x80x83(1)
where a, b1, b2, c, d, f, n are constant values.
Furthermore, in the above-described method, when bending with a forming die and a forming punch, the forming die having a flange forming portion formed parallel to the horizontal flat forming portion on the other end of the inclined flat forming portion of the forming die, and the forming punch having a flange forming portion formed parallel to the flange portion of the forming die on one end of the inclined flat forming portion of the forming punch, the method uses the forming die that has predetermined xcex8p and H so as to have xcex94W within an acceptable range, xcex94W being obtained as a function xcex94W of xcex8p, H, Lf, TS, t in advance,
where TS denotes tensile strength of a metal plate; t, a thickness of the metal plate; xcex8p, an angle formed between the horizontal flat forming portion of the forming die and the inclined flat forming portion; H, a vertical distance between the flange forming portion and the horizontal flat forming portion of the forming die; and Lf, a length of a flange portion.
It is preferable that said xcex8p is set to be 110xc2x0 or more, and H is set to be 40 mm or more.
A formula (2) given below is used as a function of
xcex94W=axc2x7(TS/t)xc2x7H2xe2x88x92bxc2x7(xcex8pxe2x88x9290)n0xc2x7H(n1xc2x7TS/t+n2)+(cxc2x7H+d)xc2x7Lfxe2x80x83xe2x80x83(2)
where a, b, c, d, n0, n1, n2 are constant values.
According to a method of bending a metal plate of the invention, during the bending process, a metal is bent in an arch shape round a bent forming portion of a forming die, and the arch portion abutting with an inclined flat forming portion of the forming die and an inclined flat forming portion of the forming punch simultaneously for a certain period of time is crushed in the final phase of forming, whereby a moment opposite to a bending moment causing an angle change defect or curvature to the formed member is generated, thereby enabling to suppress defects on shapes. Moreover, forming can be performed by using a press or forming metal die with ease, thereby having superior productivity.
Other and further objects, features and advantages of the invention will appear more fully from the following description.